Ab initio simulation of permanent densification in silica glass

To clarify the microscopic structure of densified $\mathrm{Si}{\mathrm{O}}_2$ glass, we have conducted ab initio molecular-dynamics simulations on the decompression process of $\mathrm{Si}{\mathrm{O}}_2$ glass in its relaxed state from high pressures up to 40 GPa. When decompressed from high pressures above at least 15 GPa, the density and structure always converge to those of densified glass, while the coordination number of silicon decreases to four rapidly. This is in good agreement with previous experimental studies and strongly suggests that densified glass behaves as a high-pressure polymorph of $\mathrm{Si}{\mathrm{O}}_2$ glass. In comparison to ordinary glass, although the coordination number of densified glass is almost the same, the size of an intermediate-range network consisting of $\mathrm{Si}{\mathrm{O}}_4$ tetrahedra is smaller. Detailed analyses clarify that $\mathrm{Si}{\mathrm{O}}_4$ tetrahedra in densified glass are deformed and the Si-O bonds are less covalent.

Figure 1

The pressure dependence of density and average coordination number of silicon of $\mathrm{Si}{\mathrm{O}}_2$ glass. Colored symbols represent the data on decompression and the data of ordinary glass obtained by ab initio simulations. The equations of state of densified and sixfold-coordinated glasses measured in previous experiments are also shown for comparison [4, 33]. Gray symbols represent the data of coordination number obtained by x-ray diffraction measurements on compression [22, 23].

Figure 2

The structure factor of ordinary and densified $\mathrm{Si}{\mathrm{O}}_2$ glasses. The structure factor measured with an x-ray diffraction method is also shown for comparison [5, 21].

Figure 3

The pressure dependence of abundance of fourfold-, fivefold-, and sixfold-coordinated silicon in $\mathrm{Si}{\mathrm{O}}_2$ glass (${}^{\left[4\right]}\mathrm{Si}$, ${}^{\left[5\right]}\mathrm{Si}$, and ${}^{\left[6\right]}\mathrm{Si}$ in the figure). Pentagons, hexagons, squares, and circles represent the data on decompression from 10, 15, 20, and 40 GPa obtained by ab initio simulations, respectively.

Figure 4

The ring distribution of ordinary and densified $\mathrm{Si}{\mathrm{O}}_2$ glasses. In the right side of the figure, constituent atoms of five- and six-membered rings are shown in red and blue, respectively.

Figure 5

The bond overlap population between silicon and oxygen and the O-Si-O bond angle distribution of ordinary and densified $\mathrm{Si}{\mathrm{O}}_2$ glasses. The bond overlap population indicates the strength of the covalentlike bonding.